In most commercial network interfaces, messages between computers are initiated at the operating system or application program level of the sending computer. For example, if a request to read information is received by one computer from another, the first computer processes the request and initiates a reply. It is usually the application program, or operating system, which is imaging the data requested that actually initiates the reply message.
In such systems, a received message requiring a reply requires involvement of the host computer operating system, at a minimum, to process the received message to determine what data should be sent in the reply, and to generate an appropriate transmit request to be processed by the network interface. These operations involve a substantial amount of overhead on the host computer.